President Barack Obama, center, followed by Secretary of State John Kerry, left, and Defense Secretary Ash Carter right, walks to a podium to speak to media after a meeting of his National Security Council (NSC) at the State Department in ... more >

A pessimistic President Obama warned Russia Thursday night to honor a negotiated cease-fire in Syria but said there are “plenty of reasons for skepticism” that fighting in the bitter civil war won’t end.

“None of us are under any illusions,” Mr. Obama said after emerging from a meeting with his national security team at the State Department. “We’re all aware of the many potential pitfalls, and there are plenty of reasons for skepticism.”

The U.S. and Russia agreed earlier this week to a “cessation of hostilities” between the Syrian government of Bashar Assad and groups fighting the regime. The deal, to go into effect Friday night, excludes the Islamic State and the al-Nusra Front Islamist militant groups.

“History would judge us harshly if we did not do our part in at least trying to end this terrible conflict with diplomacy,” the president said. “If implemented — and that’s a significant ‘if’ — the cessation could reduce the violence and get more food and aid to Syrians who are suffering and desperately need it. It could save lives.”

The 66-nation U.S.-led coalition continues to make progress fighting the Islamic State in Iraq and Syria, Mr. Obama said. But he said the only way to defeat the militant group permanently “is to end the chaos and the civil war that has engulfed Syria.”

“Even under the best of circumstances, we don’t expect the violence to end immediately,” Mr. Obama said, noting that the militant groups aren’t part of the cease-fire. “We are certain there will continue to be fighting.”

He said whether humanitarian aid reaches civilians in Syria will depend on large part whether Russia and its allies “live up to their commitments.”

“The coming days will be critical and the world will be watching,” he said.

The military advisers, including Defense Secretary Ashton Carter, outlined the coalition’s next phase of fighting in Iraq, which includes an effort to retake Anbar province and the city of Mosul.